State-of-the-art lithium ion batteries commonly use electrolytes containing lithium hexafluorophosphate (LiPF6) as solute and mixtures of cyclic carbonates and linear carbonates as solvents. Ethylene carbonate (EC) is the indispensable cyclic carbonate for the formation of stable solid electrolyte interface (SEI) at the surface of the negative electrode so that good battery performances can be achieved or enhanced, especially the long cycle life.
However, in many cases the SEI protection from conventional electrolytes with simple formulations such as LiPF6 in mixtures of EC and linear carbonates is insufficient in lithium ion batteries where the negative electrode materials are carbonaceous materials including graphite carbons and non-graphite carbons, for example inexpensive natural graphite (a kind of graphite carbon) and hard carbon (a kind of amorphous non-graphite carbon), which exhibits a higher initial discharge capacity but quickly loses capacity in subsequent cycles.
On the other hand, EC has a high melting point, at about 36-38° C., which limits the performance of lithium ion batteries containing EC-based electrolytes in low temperature applications. The addition of a large amount of co-solvents with low viscosity and low melting points such as linear carbonates and carboxylate esters can improve cell performance at low temperatures. Unfortunately, such co-solvents have low boiling points and are highly flammable, which present safety issues and problems in high temperature applications. Thus propylene carbonate (PC) which has a structure similar to that of EC has been considered to fully or partially replace EC so as to reduce the amount of other co-solvents in the electrolytic solutions because PC remains in the liquid state over a wide temperature window from −55° C. to 240° C. However, LiPF6—PC based electrolytes are not compatible with graphite electrode in lithium ion batteries due to the exfoliation of graphite structure by PC intercalation.
Therefore, one or more other chemical compounds, either organic or inorganic, are needed in electrolytes to form better SEI films on the disordered graphite electrodes or other carbonaceous electrodes to prevent or reduce further decomposition of solvent molecules over long service lives and to prevent the intercalation of PC into the graphite structure, so as to improve the performance of lithium ion batteries using natural graphite or hard carbon as negative electrode and containing PC in the electrolytes.
Sanyo Electric Co., Ltd. disclosed in JP 04095362 and EP 490048 the use of vinylene carbonate (VC) as co-solvents in electrolytes for lithium ion batteries. U.S. Pat. No. 5,626,981 by Bernard Simon and Jean-Pierre Boeuve of SAFT disclosed the use of vinylene carbonate and its derivatives as additives in an amount from 0.01% to 10% by weight in electrolytic solutions to help generate a passivation layer on the carbonaceous anode before any intercalation of solvated lithium ions. The passivation layer constitutes a physical barrier preventing the intercalation of solvent molecules surrounding the lithium ions and thus the lithium ion could penetrate into the carbon by itself but the exfoliation was prevented.
Seiji Yoshimura, et al. of Sanyo Denki disclosed in JP 04087156 that lithium batteries using electrolytic solvents having unsaturated C—C bonds such as vinylethylene carbonate (VEC) had low self-discharge. Minoru Kotato et al. of Mitsubishi Chemical Corporation disclosed in WO 2000/079632 that the solvents containing vinylethylene carbonate in an amount from 0.01 to 20% by weight minimized decomposition of the electrolytes and could provide a high capacity as well as excellent storage properties and cycle life performance.
Yoshinori Toyoguchi, et al. of Matsushita Electric Industrial Co., Ltd. disclosed in JP 62290072 that 1,3-dioxolan-2-ones having Cl- and/or F-substituents at the 4- and/or 5-positions were used as electrolytic solvents for secondary batteries. Roderick S. McMillan and coworkers disclosed in WO 9815024 that fluoroethylene carbonate (FEC) is capable of forming a very stable passivation film that is insoluble in the electrolyte, which substantially improved secondary battery efficiency and high capacity retention.
Kang Xu and coworkers reported in Electrochemical and Solid-State Letters (2002), 5(11), A259-A262 that lithium bis(oxalato)borate (i.e. LiBOB) could stabilize graphitic anode materials in electrolytes containing propylene carbonate (PC) while supporting reversible Li ion intercalation/de-intercalation, a behavior not yet observed for other electrolytic salts. The exfoliation of graphite in PC was prevented.
G. Chung, et al. reported in Journal of Electrochemical Society (2000), 147(12), 4391-4398 that the energy level of the lowest unoccupied molecular orbital (LUMO) energy of a compound's molecule could predict its readiness to be electrochemically reduced at the negative electrode. This semi-empirical rule has been developed to facilitate the screening process of searching for potential SEI additives and it is very helpful. It is believed that a molecule with a lower LUMO energy will be a better electron acceptor and hence more reactive at the surface of the negative electrode.